I should have posted here before ordering parts!

The kit I ordered from FCP (https://www.fcpeuro.com/products/bmw...17107544668kt1) includes something called the Engine Coolant Reservoir Bracket. Is that the holding plate you are referring to? I didn't order a replacement expansion tank.

Some of the parts are original OEM BMW, the radiator itself is Nissens.

I did also order a replacement coolant level sensor (https://www.fcpeuro.com/products/bmw...r-17137553919a), because (despite my slow leak) the sensor seems to occasionally give false readings.

I did watch a video ahead of time and realize that I'd be unplugging the trans cooler, and that there was a thermostat inside it, but I didn't order a replacement... It looked like a little cylinder that just lives in-line in the cooler hose connection, bathed in oil all the time... I didn't think it would be a problem.

Even though the kit comes with Zerex, I also ordered OEM coolant ( https://www.fcpeuro.com/products/bmw...ze-82141467704 https://www.fcpeuro.com/products/bmw...ze-82141467704)

I don't know what the O-rings shown in the kit go with, can you tell by looking at them?

I just came off another job on this car where I bought new parts and that made things worse, so I was trying to just replace the part I know is bad without trying to swap a bunch of stuff 'just because'. Maybe that is not the correct approach in this case?

It sounds like you recommend changing:
- the trans cooler t-stat
- The seals for the trans cooler (Do you know what PNs the trans cooler seals are? )

You also mentioned
-coolant lines (am I correct in thinking those are hard plastic lines under the intake which I won't necessarily be accessing if I just stick to replacing the radiator?)
-Water pump (eek, I guess I could do this as well)

What about the engine t-stat?